The present invention relates to an information recording carrier such as an optical disc and a manufacturing method thereof.
Conventionally, there have been disc information recording carriers for recording readable information and for reading the recorded information, and in recent years, the density of disc information recording carriers has grown considerably. Information recording carriers encompass a variety of discs such as magnetic discs, optical discs, electrostatic capacity discs and the like; in particular, the development in optical disc has been remarkable.
The opportunities for recording important information onto these high-density information recording carriers have increased with the recent increasing in performance of personal computers. However, security problems still remain.
These high-density information recording carriers have the major advantage in that large amounts of information can be recorded on one disc. However, they have a disadvantage of easily being duplicated with commercially available production equipment. In particular, the original disc of a high-density information on people's property such as customers' bank deposit, electronic money is liable to be stolen, and copied (counterfeited) in large numbers for distribution to interested parties. Complete prevention of such crime is impossible. So, there always remains the risk of violation of customer privacy and a sense of unease in financial dealings. This, in return, affects the stability of business dealings, and can result in a loss of confidence financial dealings.
Accordingly, a method for preventing the unlawful duplication of high-density information recording carriers loaded with important information, has been required. As means to solve this problem, there have been proposed various copy protection systems using ciphers. These systems protects the information against electronic copying. The advance of ciphers has made it difficult to copy the information. In response to this, a physical method come to be used for unlawful copying. In this method, the disc is delaminated, exposing the information surface, and thus the information is transferred to another disc. The disc is used as a mold to transfer the signal onto another flat plastic disc. This mold can be repeatedly used, making it possible to make numerous signal patterns exactly the same as the original. Thus, apart from electronic copying, there also is a physical method for copying the entire information, and there is still no means for preventing this.
FIG. 1 illustrates the construction of an optical disc. As shown in FIG. 1, an optical disc A is constructed by sequentially superposing a recording layer 2 (a reflective film), and a protective layer 3 on a substrate 1. The signal surface on the substrate 1 includes fine pattern rows 1A. In optical disc such as disc A, the interface between the substrate 1 and the recording layer 2 can be delaminated. In general, the protective layer 3 is made to achieve sufficient adhesion with the recording layer 2, and is hard enough to function as a protective layer. The interface between the substrate 1 and the recording layer 2, however, has weak adhesion, and the surface of the substrate 1 can be exposed by delaminating the disc carefully with an adhesive tape. The recording layer 2 is formed on the substrate 1 by an evaporation or sputtering method or the like, the latter of which while achieving good adhesion, is weak enough to allow the signal surface to be exposed with an adhesive tape.